1. Field of the Invention
The present invention relates generally to the art of vehicle clutch systems, and more particularly to clutch systems employing both a master cylinder and a slave cylinder. Still more specifically, the present invention relates to a hydraulic master-slave clutch system which also includes a slave cylinder sensor that generates electrical signals that can be used to control a number of the vehicle's functions that are dependent upon clutch position and wear.
2. Description of the Prior Art
It is well known in the motor vehicle art that a hydraulic cylinder can be employed for clutch release. It is also known to employ a master hydraulic cylinder coupled respectively to the vehicle clutch pedal and to a slave hydraulic cylinder. The slave cylinder in turn is coupled to the throw-out bearing of the vehicle clutch.
A typical hydraulic clutch is shown in Hodkinson's U.S. Pat. No. 4,267,697 issued May 10, 1981 for "Hydraulic Master Cylinder." Illustrative master-slave cylinder combinations for clutch actuation are shown in the following U.S. patents: Maurice, U.S. Pat. No. 3,307,667, issued Mar. 7, 1967 for "Declutching Device for Obviating Wear Adjustment". Buchanan, et al., U.S. Pat. No. 3,406,800 issued Oct. 22, 1968, for "Hydraulic Clutch Wear Warning Device"; and Espenschied, U.S. Pat. No. 3,752,282 issued Aug. 14, 1923, for "Clutch with Plural Fluid Releasing Means." In each of the latter three patents a master cylinder is located in the vicinity of the clutch pedal and is actuated by a push rod connected thereto. Hydraulic tubing connects the master cylinder to a slave cylinder located in the vicinity of the clutch. The slave cylinder is connected to the clutch mechanism, e.g., by a rod extending from the slave cylinder. The combination of a master and a slave cylinder for clutch release has proven to be advantageous because it permits greater flexibility in vehicle design, and combination systems are now employed in a number of commercial vehicles.
Such use of two cylinders increases the number of components which can malfunction. In co-pending, commonly assigned patent application Ser. No. 710,593 filed Mar. 11, 1985, a master-slave clutch system is described. The system of the aforementioned application includes an improvement over the other prior references described above in that a system is included for preventing ignition of the vehicle unless positive actuation of the slave cylinder has occurred to release the clutch plate. The systems described in the application include Hall effect sensors, hydraulic fluid pressure sensors, bar code readers, etc. The application describes systems in which the sensors may be located in either or both of the master and slave cylinders, it being preferred to always include such a sensor in the slave cylinder.
The co-pending application describes circuitry connected to the sensors which will prevent ignition of the motor vehicle unless a component of the slave cylinder has moved a predetermined distance. Using such a system, malfunctions of the master-slave cylinder system cannot lead to lurching of the vehicle in the event the clutch pedal is depressed but the clutch itself has not been released.
While the aforementioned co-pending application describes a system which represents a significant improvement over prior art master-slave systems, several problems are encountered utilizing same. First, as wear of the clutch plate increases, the spring forces acting upon the throw-out bearing of the clutch and coupled to the slave cylinder by suitable linkages, will cause a gradual shift of the position of the piston of the slave cylinder. As wear continues, the degree of such shift will increase. Even though the amount of wear is not sufficient to require repair of the vehicle, the movement of the piston of the slave cylinder may result in inaccurate sensing by certain of the systems described in the co-pending application. False signals may therefore result, even though many miles of additional driving may be possible without adjustment to or replacement of any clutch components.
Another problem inherent in most prior art master-slave cylinder combinations is that clutch wear, if it exceeds a certain amount, may be dangerous to the clutch components or even to the slave cylinder components if not promptly detected. One solution to this problem is suggested in the aforementioned Buchanan, et al., U.S. Pat. No. 3,406,800. In this patent, a biasing spring couples a linkage arm between the push rod of the slave cylinder and the throw-out bearing of the clutch and a pair of electrical contacts are provided inside the slave cylinder. A second biasing spring is interposed between the two contacts. This patent recognizes the movement of the slave cylinder piston as wear increases and arranges the internal components of the slave cylinder so that when the force of the first biasing spring is overcome (because of clutch wear), the electrical contacts will come into close proximity and finally contact one another to ignite a warning lamp on the vehicle dash. The lamp arrangement with the electrical contacts should be arranged so that the lamp will ignite prior to the time that damage has occurred because of wear to either the clutch or slave cylinder components. The prior art and co-pending application further suffer from the shortcoming of only performing either the function of checking clutch wear or of ensuring clutch release prior to ignition.
It would represent a significant advance in the art to provide a master-slave cylinder combination which overcomes the disadvantages of the prior art master-slave systems and which additionally overcomes the problems and shortcomings noted with regard to the co-pending application. More specifically, it would be advantageous to have a piston rod position sensor system designed for use with at least the slave cylinder so that movement of the slave cylinder piston due to clutch wear could be dealt with and compensated for by the system which senses whether sufficient movement of the slave cylinder push rod has occurred. Likewise, it would also be advantageous to use a system to control other vehicle systems that typically depend on piston position for enabling or disabling. Moreover, it would be advantageous to include in such system a method for determining on a continual basis the amount of clutch wear which has occurred and which would not depend upon any limit being reached. The latter feature would enable mechanics to evaluate the degree of clutch wear during any routine analysis of the vehicle and therefore conduct advanced evaluations of the timeliness of clutch repair needs.
It is well known in the hydraulic cylinder art that conductive and resistive elements may be used to determine the position of a piston within a cylinder. Typical of these systems is that disclosed in Johnston, et al., U.S. Pat. No. 3,726,191, issued Apr. 10, 1973 for "Electrically Controlled Hydraulic system and Transducer Therefore." Johnston illustrates a hydraulic cylinder of the kind used in construction or earth moving equipment. The voltage between a contact point on the piston and one end of the cylinder is compared to the voltage between the two ends of the cylinder. These voltages are measured on the same elongate piece of conductive material with a uniform unit resistance. By comparing the two measured voltages, an electrical control system determines the relative resistances and thus the position of the piston within the cylinder. A similar system is disclosed in McCormic's U.S. Pat. No. 3,720,118, issued Mar. 13, 1973 for "Integrated Servo Actuator." A conductive bimetal bar and brush assembly produce a feedback signal that is an electrical readout of the actuator's position. Other related devices incorporating conductive strips within the cylinder unit are shown in Buxton, et al., French Pat. No. 1,382,342, issued Nov. 9, 1963 for "Fluid Cylinder Positioning Systems," Ward's U.S. Pat. No. 3,412,391, issued Nov. 19, 1968 for "Pressure-Fluid-Operated Devices and Means for Indicating the Condition Thereof," and Foxwell's U.S. Pat. No. 4,386,552, issued June 7, 1983 for "Power Cylinder with Internally Mounted Position Indicator."
It is also known in the hydraulic cylinder art that optical systems may be used to determine the position of a piston within a cylinder.
An example of this type of system is the co-pending, commonly assigned application Ser. No. 710,593, cited above. One of the alternate embodiments of the invention disclosed in that application utilizes an optical bar code to determine the positions of the rods and pistons in both the master and slave cylinders.